Most reconstitution errors are not chemistry errors — they are arithmetic errors made under time pressure. Here is the maths laid out cleanly, with the three traps that catch everyone.
A reconstitution calculation has exactly three inputs and one output, and it still goes wrong constantly. The inputs are the labelled peptide mass in the vial, the volume of diluent you add, and the mass you want per draw. The output is a syringe volume. Everything else is unit conversion.
The core equation
Concentration equals vial mass divided by diluent volume. Draw volume equals target mass divided by concentration. That is the entire calculation. A 10 mg vial reconstituted with 2 mL of bacteriostatic water gives 5 mg/mL; a 500 mcg target therefore needs 0.1 mL.
| Vial | Diluent added | Concentration | Volume for 250 mcg | Volume for 1 mg |
|---|---|---|---|---|
| 5 mg | 1 mL | 5 mg/mL | 0.05 mL | 0.20 mL |
| 5 mg | 2 mL | 2.5 mg/mL | 0.10 mL | 0.40 mL |
| 10 mg | 2 mL | 5 mg/mL | 0.05 mL | 0.20 mL |
| 10 mg | 3 mL | 3.33 mg/mL | 0.075 mL | 0.30 mL |
| 20 mg | 2 mL | 10 mg/mL | 0.025 mL | 0.10 mL |
Trap one — insulin syringe units are not millilitres
A U-100 insulin syringe is graduated in 100 units per millilitre. One unit is 0.01 mL. Read that graduation as a volume, not a dose. The single most common bench error in Australian labs is treating "10 units" as though it carries a fixed mass across different vial concentrations — it does not.
Trap two — labelled mass is not net peptide
Counterion salt and residual moisture mean a 10 mg vial typically contains somewhat less than 10 mg of net peptide. If your work is sensitive to that difference, use the peptide-content figure from the certificate rather than the label weight, and record which one you used.
Trap three — diluent volume drift
Adding "about 2 mL" is not adding 2 mL. Use a syringe you can read to 0.05 mL, add it slowly down the vial wall rather than onto the cake, and write the actual delivered volume on the vial the moment you finish. A concentration you cannot reproduce is a concentration you do not have.
Reconstitution FAQ
Bacteriostatic or sterile water?
Bacteriostatic water contains benzyl alcohol and is the standard choice when a reconstituted vial will be drawn from repeatedly over days or weeks. Sterile water is appropriate for single-session use.
Why must diluent run down the vial wall?
Directing the stream onto the lyophilised cake causes local shear and foaming. Peptides are surface-active and foam means denatured material at the air–liquid interface.
Should I shake the vial to dissolve faster?
No. Swirl gently and wait. Shaking introduces the same interfacial denaturation problem as jetting diluent onto the cake.
